Web pages are electronic documents that can include textual, graphic, video, and audio content. Most Web pages are generated using the HyperText Mark-up Language (HTML), although the pages can include data encoded according to other formats, e.g., Moving Picture Experts Group (MPEG), Joint Photographic Experts Group (JPEG), Graphics Interchange Format (GIF), and so forth. The most common way to access a Web page is by using a Web browser. Typically, the pages are transferred from servers to recipient systems (the clients) using the HyperText Transfer Protocol (HTTP). HTTP is an application level protocol that is layered on top of the TCP/IP protocols.
In the Internet/intranet communications, the “effective” throughput of communication paths between servers and clients can vary greatly. The effective throughput depends on transmission rates, number of “hops,” error rates, latencies, and so forth. Because servers and clients can be connected via a wide range of network technologies, the effective throughput can span several orders of magnitude. This means that a Web page that includes rich content designed for a high throughput path may not always be inappropriate for use by client systems that have slow connection to network servers over paths with much lower throughput.
Some existing systems permit the use of a specialized tag that allows an HTML-coded Web page to specify the use of two versions of a given image. The browser initially loads a low-resolution version of the image; then automatically loads a high resolution version to replace the low-resolution image.